The Grebulons are a race that only appear in the novel Mostly Harmless.
They traveled across the galaxy in a sub-light ship and as such were in stasis during their trip. Their ship's main computer was the victim of a passing meteorite and while they survived in stasis, their memories, most of their central mission module, and the part of the ship that should have been able to detect that it had been struck by a meteorite were destroyed. All that it was able to resconstruct of their mission was "...Land...," "...safe distance..." and "...monitor it, and..."
They became enamored with Astrology and invited the alternate Earth's Tricia McMillian to Rupert to map out their astrological horoscopes and offered her reheated chicken nuggets.
In a misguided attempt the Grebulon Leader ordered Earth destroyed before it could rise into Capricorn. As a classic Taurus, this was very ominious for him. He ended up having a bad month indeed as there was no longer transmissions from Earth to monitor.
While trying to save Random from the Earth's destruction, Trillian reveals that the Grebulons are a missing reconnaissance fleet from the war that she was meant to cover, but never happened.